The invention relates to an apparatus for contactless guiding of webs of material, in particular metal strips, by means of a gas medium, with nozzle boxes which are disposed above and below and spaced apart from the web of material and in each case comprise a nozzle outlet plane and between which are provided gas discharge channels above and below the web of material.
Apparatuses of this kind are used in particular for heating or cooling strip-like or web-like material. Air or inert gas which is blown onto the web of material is used predominantly as the gas medium.
There is already known (German Auslegeschrift No. 25 21 017) the possibility of guiding webs of material, in particular metal strips, with a thickness of 0.1 mm or more uniformly and in plane relationship through a gap between oppositely directed nozzle boxes. In this case the nozzle boxes extend at an angle to the direction of transport of the web of material, and the longitudinally or transversely directed nozzles are offset from each other. The nozzle boxes, which are disposed near a web of material, are separated from each other by gas discharge channels for conducting away the gas medium which is blown through the nozzles onto the web of material. In these gas discharge channels are inserted throttle valves for regulating the gas discharge.
With this known apparatus, the guiding of metal strips with a thickness of 0.3 mm or more can no longer be carried out satisfactorily. With strips made of special metal alloys, e.g. some copper alloys, the critical strip thickness may be below 0.3 mm.
Advantageous sinusoidal guiding of metal strips by the apparatus cannot be achieved in this case. The metal strip of appropriate thickness or stiffness leaving the apparatus has, on the contrary, an inadmissibly high proportion of longitudinal folds and other uneven features, so that a high proportion of scrap must be expected.
There is further known an apparatus (German Patent No. 33 18 861) by which webs of material, in particular aluminum strips, can be guided through in floating relationship for the purpose of annealing. In this case nozzle boxes are used which extend at an angle to the direction of transport of the webs of material and which, to form air cushions, each comprise two longitudinal slots or rows of holes whose directions of blowing are inclined towards each other. These nozzle boxes are arranged in pairs above and below the web of material. One of these pairs of nozzle boxes disposed above the web of material is then followed in the direction of transport by another pair of nozzle boxes below the web of material. This pair is then followed in the direction of transport by another pair of nozzle boxes above the web of material. This means that there are large gaps between those pairs of nozzles which are disposed above or below the web of material. Consequently, heating or cooling of the web of material relative to the section of treatment can only be relatively insignificant. To achieve a high plant yield, therefore, a relatively long section of treatment is required with corresponding technical elaborateness.
With the known apparatus, in addition, the carrying capacity of the gas jets inclined towards each other and exiting from the nozzle boxes is very low, so that this apparatus cannot be suitable for relatively thick and heavy metal strips. Although the nozzle boxes have to be constructed at great expense with this apparatus, the sinusoidal path which is the optimum for the supporting and guiding effect cannot be achieved with this known apparatus.